The DESI project

Andreu Font-Ribera


In 2015, IFAE joined the Dark Energy Spectroscopic Instrument (DESI), an international project led by LBNL (USA). Together with groups at ICE (Institut de Ciències de l’Espai), CIEMAT (Centro de Investigaciones Energéticas, Medio Ambientales y Tecnológicas) and Universidad Autónoma de Madrid (UAM), IFAE designed and built the Guiding, Focus and Alignment cameras (GFA), a key hardware component of DESI.

INTRODUCTION

In May 2021, DESI started a five-year campaign to measure accurate redshifts of 40 million galaxies and quasars over 14 000 square degrees, a third of the sky. To do this, it is using a new multi-fiber spectrograph at the Mayall 4-m telescope at Kitt Peak National Observatory, in Arizona (US), equipped with 5000 robotically actuated fibers. By the end of 2021, DESI had already collected the largest spectroscopic dataset to date.

2025 Activities

In March 2025, DESI made publicly available its first Data Release (DR1), including 18.7 million new spectra of galaxies and quasars observed from May 2021 through June 2022. This very rich dataset is described in this manuscript (Ref 1), submitted for publication to the Astropysical Journal.

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Figure 1: This slice of the DESI data maps celestial objects from Earth (center) to billions of light years away. Among the objects are nearby bright galaxies (yellow), luminous red galaxies (orange), emission-line galaxies (blue), and quasars (green). The large-scale structure of the universe is visible in the inset image, which shows the densest survey region and represents less than 0.1% of the DESI survey’s total volume.

At the same time, DESI also presented cosmological analyses from its second Data Release (DR2), using observations from the first three years of observations. In particular, it presented the measurements of Baryon Acoustic Oscillations (BAO) from the clustering of galaxies and quasars (Ref 2) and from the clustering of absorption features in the Lyman alpha forest (Ref 3). BAO measurements can be used to accurately study the expansion history of the Universe, and in combination with measurements of the Cosmic Microwave Background (CMB) they provide some of the most stringent tests to the standard model of cosmology (Ref 2).
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Figure 2: A deep view of a slice of DESI’s first three years of data, the largest 3D map of our universe ever made. Credit: DESI Collaboration and KPNO/NOIRLab/NSF/AURA/R.

The Observational Cosmology group at IFAE contributed actively to these results, in particular in the BAO measurement using the Lyman alpha forest. The validation of this measurement using synthetic datasets was presented in a publication (Ref 4) led by Laura Casas, PhD student at IFAE. The impact of the DESI DR2 BAO measurements has been very large, and Ref 2 has already received more than 800 citations in less than a year. Andreu Font-Ribera was also asked to present these results in a CFA Colloquium at Harvard.

Finally, the cosmology group at IFAE also hosted a DESI workshop in September of 2025. This workshop, attended by 15 members, focused on the preparation of future analyses of the Lyman alpha forest that go beyond the BAO measurements presented above.

References